docs: def filter_content_types(self, content_type_qs):
          """ Filter the content types selectable to only event subclasses """
          valid_ct_ids = []
          for ct in content_type_qs:
              model = ct.model_class()
              if model and issubclass(model, EventBase):
                  valid_ct_ids.append(ct.id)
          return content_type_qs.filter(pk__in=valid_ct_ids)